Citric Acid-Based Carbon Dots and Their Application in Energy Conversion

Carbon dot (CD) is a general term that encompasses a plethora of organic materials obtained from different reagents and different synthetic routes. Beyond this variety, carbonization has been considered for a long time as the essential process whereby completely different precursors converge into the formation of nanosized particles with resembling optical properties. In particular, CDs' outstanding fluorescence emission is frequently attributed to the existence of large aromatic structures with extended conjugation. Nevertheless, thanks to the contribution of an increasing number of publications, nowadays the impact of these structures on the fluorescence emission has been considerably downsized. In some cases, their existence is even highly questionable. In return, in these years, more solid interpretations have been provided, according to which the chemical and emissive nature of CDs is strictly specific to the combination of synthetic precursors employed. Moreover, researchers have become more aware of the complexity of these systems, which contain not only nanoparticles but also single molecules, oligomers, and aggregates with their own fluorescent behavior. Therefore, we dedicate the first part of this review to the individual examination of different classes of citric acid-based CDs as a benchmarking CD class. A classification based on the precursors is meant to help the reader to find a common thread between the structural and optical properties of CDs. In the second part, we discuss the employment of citric acid-based CDs for energy conversion applications. This includes their exploitation as components of light-emitting diodes (LEDs), as photocatalysts for hydrogen generation and pollutant degradation, and as additives in several types of organic photovoltaic devices.